HYBRID BOOSTED OVERFIRE AIR SYSTEM AND METHODS FOR NOx REDUCTION IN COMBUSTION GASES

ABSTRACT

A boiler incorporates an overfire air injection system for reducing NO x  emissions. The boiler comprises a combustion device including a plurality of main burners supplied with fossil fuel and air for burning in a combustion zone, where the burners produce flue gases that flow from the combustion zone into a burnout zone. The boiler further comprises at least one overfire air injector for supplying overfire air to the combustion device and at least one booster overfire air injector for supplying high-pressure air to the combustion device.

BACKGROUND OF THE INVENTION

This invention relates generally to enhancements to overfire air for NO_(x) control. In particular, the invention relates to Overfire Air for NO_(x) control that comprises overfire air and booster fans to further supply overfire air.

This disclosure is related to other overfire air patents only because it applies air staging to reduce NOx emissions, which is common to all OFA systems. Some related patents include: U.S. Pat. No. 05,727,480, U.S. Pat. No. 06,318,277, U.S. Pat. No. 06,325,003, U.S. Pat. No. 06,865,994, U.S. Pat. No. 07,004,086, U.S. Pat. No. 07,047,891. The contents of U.S. Pat. No. 06,865,994 and U.S. Pat. No. 07,004,086 are incorporated by reference.

One of the major problems in modern industrial society is the production of air pollution by a variety of combustion systems, such as boilers, furnaces, engines, incinerators and other combustion sources. One of the oldest recognized air pollution problems is the emission of oxides of nitrogen (NOx). In modern boilers and furnaces, NOx emissions can be eliminated or at least greatly reduced by the use of overfire air (OFA) technology. In this technology, most of the combustion air goes into the combustion chamber together with the fuel, but addition of a portion of the combustion air is delayed to yield oxygen deficient conditions initially and then to facilitate combustion of CO and any residual fuel.

OFA systems rely on the momentum of the OFA jets to provide effective mixing with the flue gas stream. For a given OFA mass flow rate, penetration into the flue gas stream and the rate of mixing is controlled by the size and number of individual OFA jets and by their corresponding velocity. Higher velocities and small openings result in faster mixing rates, while larger openings lead to better penetration of the air into the flue gas stream. In practical combustion systems, the maximum OFA velocity which can be applied is typically limited by the pressure inventory available in the combustion air supply system, such that mixing rate and jet penetration cannot be controlled independently.

When the secondary air source pressure is too low, high-pressure boost fan(s) can be used to supply high-pressure air to the OFA injectors. Fully boosted OFA systems are costly and sometimes difficult accommodate due to weight or volume limitations in the boiler superstructure. One version of boosted overfire air is called rotating overfire air (ROFA), a technology supplied by a Mobotec.

Current OFA systems can apply some passive or active methods for controlling near field mixing. In these systems, large-scale flow structures may be generated that significantly reduce mixing effectiveness near the injector outlet. This leads to the need for higher airflow velocities that may not be attainable due to pressure inventory limitations.

BRIEF DESCRIPTION OF THE INVENTION

A boiler incorporates an overfire air injection system for reducing NO_(x) emissions. The boiler comprises a combustion device including a plurality of main burners supplied with fossil fuel and air for burning in a combustion zone, where the burners produce flue gases that flow from the combustion zone into a burnout zone. The boiler further comprises at least one overfire air injector for supplying overfire air to the combustion device and at least one booster overfire air injector for supplying high-pressure air to the combustion device.

A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion, is also within the scope of the invention. The method comprises providing a fossil fuel to a combustion device. The combustion device and as embodied by the invention, includes a plurality of main burners supplied with the fossil fuel and air, for burning fossil fuel and air in a combustion zone. The burning producing flue gases that flows from the combustion zone into a burnout zone. Overfire air is provided to the combustion device through at least one overfire air injector at a first pressure. Additionally, as embodied by the invention, booster overfire air is supplied through at least one booster overfire air injector at a second pressure. The overfire air from the at least one overfire air injector is at a first pressure is at a lower pressure than the second pressure from the at least one booster overfire air injector.

These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawing, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic, part sectional view of a combustion device of a fossil fuel-fired combustion device, such as used in a fossil fuel-fired boiler or furnace, as embodied by the invention.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and “the like”, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of “up to about 25 wt. %, or, more specifically, about 5 wt. % to about 20 wt. %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 25 wt. %,” etc.). The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the colorant(s) includes one or more colorants). Furthermore, as used herein, “combination” is inclusive of blends, mixtures, alloys, reaction products, and the like.

Referring now to FIG. 1, there is a schematic representation of a fossil fuel-fired combustion device 100 such as used in a fossil fuel-fired boiler or furnace. Combustion device 100 includes a combustion zone 122 and a burnout zone 124. The combustion device 100 may also include a reburning zone 126 between the combustion and reburning zones.

The combustion zone 122 is equipped with at least one, and preferably a plurality of main burners 128, which are supplied with a main fuel, such as but not limited to, fossil fuels, through a fuel input 13, and with air through at least one air input 11 and 12. The main fuel, which can comprise suitable coal in any form, including pulverized coal, from coal hopper 1, is burned in the combustion zone 122 to form a combustion flue gas that flows upwardly from the combustion zone 122 toward the burnout zone 124, a direction referred to herein as a “downstream” direction.

Downstream of the reburning zone 126, overfire air is injected through an overfire air or OFA injector 10 into the burnout zone 124. The combustion flue gas passes through a series of heat exchangers 140, where the heat withdrawn at 24 can be supplied to a steam turbine. Further, it is possible that any solid particles can be removed by a particulate control device (not shown), such as an electrostatic precipitator (“ESP”) or baghouse. The flue gases exit the boiler or furnace at the outlet 42.

When the secondary air source pressure is too low, at least one high-pressure boost fan(s) 50 can be used to supply high-pressure air to at least one of the OFA injectors, the burners 128, and the entire combustion device 100, as embodied by the invention. The at least one high-pressure boost fan(s) 50, which, as embodied by the invention, can be provided in the form of booster overfire air injectors that can supplement the OFA, as either one of heated/hot or ambient/cold air or combinations thereof, delivered into the combustion device 100. Alternatively, separate high-pressure boost fan(s) 50 supply high-pressure air to each of the components of the combustion device, as embodied by the invention.

Overfire air is a well-known technology that is used to reduce NOx emissions in utility and industrial furnaces. Hybrid boosted overfire air combines two discrete air supply systems, boosted air and secondary combustion air, to achieve effective penetration and mixing of overfire air with combustion gas. A portion of the overfire air (OFA) is delivered to the OFA injectors as either “cold” or “hot” high-pressure air from booster fans (BOFA). The remaining overfire air is delivered to the OFA injectors from the existing “hot” secondary combustion air (HOFA) system (e.g., ducting or burner windbox). This approach is a low-cost alternative to a traditional stand-alone boosted overfire air system.

Overfire air is a well-known technology that is used to reduce NOx emissions in utility and industrial furnaces. Traditional OFA systems divert secondary combustion air from a burner windbox to the OFA injectors. The OFA supply pressure in the burner windbox or secondary air ducting, determines the maximum dynamic pressure that will be available at the OFA injector outlet. Sufficient OFA dynamic pressure ensures effective penetration and mixing of overfire air with combustion gas. In some cases, the available dynamic pressure to the OFA injector is not high enough to achieve the required penetration and mixing of the air and combustion gas. If this happens, the provision of the BOFA can assist in reducing NOx emissions.

A feature of BOFA in conjunction with OFA is that both boosted high-pressure air (BOFA) and low-pressure secondary combustion air, such as OFA., achieve air jet penetration and mixing in an overfire air system. Good jet penetration and mixing in these systems lead to effective NOx reduction and help lower CO emissions. To date, other OFA systems apply either stand-alone secondary combustion air (traditional OFA) or stand-alone BOFA for jet penetration and mixing. Mixing effectiveness in tradition OFA systems is sometimes limited by a low supply pressure. Standalone BOFA systems are costly and often cause erosion problems on the waterwalls and superheater tubes.

Hybrid BOFA will be used to reduce NOx emissions in utility boilers when the air supply pressure is too low to achieve the required mixing between the air and combustion gas. Hybrid BOFA is a low-cost alternative product to stand-alone BOFA. Hybrid boosted OFA combines both boosted overfire air (BOFA) and overfire air OFA, that can use only preheated secondary combustion air. Some features of this system comprise, but are not limited to:

(A) Cold ambient or hot preheated overfire air can be supplied at a higher than normal boost pressure, to induce the high temperature, low pressure air, and provide a desired level of penetration into, and mixing with, the boiler gases.

(B) Boosting a portion of the OFA lends to smaller fans (for OFA and/or BOFA) with a reduced weight, reduced power requirements, and lower capital cost.

(C) A reduced fan size and weight allows a fan to be mounted on a platform near the OFA injector elevation, where there is ample space, and where air duct runs to the OFA ports are relatively simple. Smaller fans can more readily be located and isolated with only minimal additional reinforcement.

(D) Provides a portion of the overfire air through separate fans will reduce the duty required of the fans and is expected to ease existing fan limitations. BOFA should permit full load operation at higher excess O₂ levels than currently possible, which could provide greater power generation at peak periods, with improved control of fly ash and CO emissions.

A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion, is also within the scope of the invention. The method comprises providing a fossil fuel to a combustion device 100. The combustion device 100, as noted above and as embodied by the invention, includes a plurality of main burners 28 supplied with the fossil fuel and air 12, for burning fossil fuel and air in a combustion zone 122. The burning producing flue gases that flows from the combustion zone 122 into a burnout zone 124. Overfire air is provided to the combustion device 100 through at least one overfire air injector 9 at a first pressure. Additionally, as embodied by the invention, booster overfire air is supplied through at least one booster overfire air injector 9 at a second pressure. The overfire air from the at least one overfire air injector 10 is at a first pressure is at a lower pressure than the second pressure from the at least one booster overfire air injector 9.

The method, as embodied by the invention, also comprises at least one of supplying overfire air to the burnout zone from the at least one overfire air injector, supplying second pressure overfire air to the burnout zone from the at least one booster overfire air injector, and supplying second pressure overfire air to the plurality of main burners from the at least one booster overfire air injector.

Moreover, the method, as embodied by the invention, can also supply second pressure overfire air to the burnout zone and the plurality of main burners from the at least one booster overfire air injector, supply second pressure overfire air as either one of heated air and ambient air or combinations thereof.

A method, as embodied by the invention, reduces nitrogen oxide (NO_(x)) emissions formed during the combustion in part by achieving air jet penetration in the combustion device and mixing in the combustion device to reduce NO_(x) emissions.

The competitive advantage that hybrid BOFA has relative to stand-alone BOFA is primarily the use of a smaller boost fan leading to:

reduced fan weight;

reduced fan power requirements;

fan mounting near the OFA injector elevation leading to simple duct runs to the OFA injectors;

Reduced steelwork reinforcement; and

Lower capital cost.

A further aspect of the invention an increased a pressure drop of secondary combustion air at the OFA supply location. This drop is achieved by removing excessive flow resistance in the airflow circuit or bypassing system unit operations such as the air heater.

While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. 

1. A boiler incorporating an overfire air injection system and a booster overfire air injection system for reducing nitrogen oxide (NO_(x)) emissions, the boiler comprising: a combustion device including a plurality of main burners supplied with fossil fuel and air for burning in a combustion zone, producing flue gases that flow from the combustion zone into a burnout zone; at least one overfire air injector for supplying overfire air to the combustion device; and at least one booster overfire air injector for supplying high-pressure air to the combustion device, wherein the overfire air from the at least one overfire air injector is at a lower pressure than the high-pressure air from the at least one booster overfire air injector.
 2. A boiler according to claim 1, wherein the at least one overfire air injector supplies overfire air to the burnout zone.
 3. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies high-pressure air to the burnout zone.
 4. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies high-pressure air to the plurality of main burners.
 5. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies high-pressure air to the burnout zone and the plurality of main burners.
 6. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies boosted overfire air as either one of heated air and ambient air or combinations thereof.
 7. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies boosted overfire air as heated air.
 8. A boiler according to claim 1, wherein the at least one booster overfire air injector supplies boosted overfire air ambient air.
 9. A boiler according to claim 1, wherein the at least one booster overfire air injector and the at least one overfire air achieve air jet penetration and mixing in the combustion system to reduce NO_(x) emissions.
 10. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion, said method comprising the steps of: providing a fossil fuel to a combustion device, where the combustion device includes a plurality of main burners supplied with the fossil fuel and air for burning fossil fuel and air in a combustion zone; producing flue gases that flows from the combustion zone into a burnout zone; supplying overfire air to the combustion device through at least one overfire air injector at a first pressure; and supplying booster overfire air injector through at least one booster overfire air injector at a second pressure, wherein the overfire air from the at least one overfire air injector at a first pressure is at a lower pressure than the second pressure from the at least one booster overfire air injector.
 11. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying overfire air to the burnout zone from the at least one overfire air injector.
 12. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying second pressure overfire air to the burnout zone from the at least one booster overfire air injector.
 13. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying second pressure overfire air to the plurality of main burners from the at least one booster overfire air injector.
 14. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying second pressure overfire air to the burnout zone and the plurality of main burners from the at least one booster overfire air injector.
 15. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying second pressure overfire air as either one of heated air and ambient air or combinations thereof.
 16. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising supplying second pressure overfire air as heated air.
 17. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10 further comprising supplying second pressure overfire air as air ambient air.
 18. A method for reducing nitrogen oxide (NO_(x)) emissions formed during the combustion according to claim 10, further comprising achieving air jet penetration in the combustion device and mixing in the combustion device to reduce NO_(x) emissions. 